The present disclosure relates to solid oxide fuel cells, and more particularly relates to a gas control valve used in solid oxide fuel cells.
A fuel cell is an energy conversion device that generates electricity and heat by electrochemically combining a gaseous fuel, such as hydrogen, carbon monoxide, or a hydrocarbon, and an oxidant, such as air or oxygen, across an ion-conducting electrolyte. The fuel cell converts chemical energy into electrical energy. A fuel cell generally consists of two electrodes positioned on opposites of an electrolyte. The oxidant passes over the oxygen electrode (cathode) while the fuel passes over the fuel electrode (anode), generating electricity, water, and heat.
The automotive industry has turned to fuel cells, particularly solid oxide fuel cells (SOFCs), to help power automobiles and reduce emissions. SOFCs are constructed entirely of solid-state materials, utilizing an ion conductive oxide ceramic as the electrolyte. A conventional electrochemical cell in a SOFC is comprised of an anode and a cathode with an electrolyte disposed therebetween. In a typical SOFC, a fuel flows to the anode where it is oxidized by oxide ions from the electrolyte, producing electrons that are released to the external circuit, and mostly water and carbon dioxide are removed in the fuel flow stream. At the cathode, the oxidant accepts electrons from the external circuit to form oxide ions. The oxide ions migrate across the electrolyte to the anode. The flow of electrons through the external circuit provides for consumable or storable electricity. However, each individual electrochemical cell generates a relatively small voltage. Higher voltages are attained by electrically connecting a plurality of electrochemical cells in series to form a stack.
The SOFC stack also includes conduits or manifolds to allow passage of the fuel and oxidant into the stack, as well as excess fuel and oxidant with byproducts, out of the stack. Generally, oxidant is fed to the structure from a manifold located on one side of the stack, while fuel is provided from a manifold located on an adjacent side of the stack. The fuel and oxidant are generally pumped through the manifolds and introduced to a flow field disposed adjacent to the appropriate electrode. The flow fields that direct the fuel and oxidant to the respective electrodes typically create oxidant and fuel flows across the electrodes that are perpendicular to one another.
Seals must be provided around the edges of the various cell stack components to inhibit crossover of fuel and/or oxidant. For example, seals are disposed between the electrodes and adjacent flow fields, around manifolds, between flow fields and cell separators, and elsewhere. One factor in establishing SOFC reliability is the integrity of these seals.
Leaks in the manifold seals, electrochemical seals, or other defects that allow air (oxygen) in can lead to the SOFC failure. When the concentration of oxygen on the anode side forms an oxidizing environment, anode oxidation can occur, creating a volume change that results in mechanical failure of the electrochemical cell. To address this problem, conventional electrochemical cells provide a continuous supply of fuel (or reformate) to continue to provide hydrogen to the anode (i.e., maintaining a reducing environment) and inhibit anode oxidation. However, during transition periods of shut down and start up, generally the reformate is not present in the fuel cell and the concentration of oxygen can be elevated, causing anode oxidation, particularly when the temperature exceeds 200xc2x0 C.
What is needed in the art is a method of protecting the anode from anode oxidation during start up and shut down periods.
A method and apparatus are disclosed herein utilizing a gas control valve to regulate and contain reducing gas flow to the anode for anode protection. A fuel cell having a gas control valve, the fuel cell comprising a fuel cell unit, an inlet and an outlet coupled to the fuel cell unit, an actuator disposed in the gas control valve, and a reducing gas supply for actuating the actuator for regulating gas to the fuel cell unit. A method of reducing anode oxidation utilizing at least one gas control valve is also disclosed herein.